The present invention is concerned with an electrophotographic printing apparatus for printing an image using an electrophotographic method, and more particularly, it relates to an exposure controlling device for the apparatus.
A conventional electrophotographic method (a so-called three level printing method) comprises the steps of forming an electrostatic latent image consisting of two electric potential levels and a nonprinting electric potential level in one exposure for developing an image in two colors. This method is disclosed in Japanese patent Laid-open print No. 48-37148. In this method, an electrostatic latent image consisting of three electric potential levels is formed by irradiating a photosensitive member with light modulated on the basis of image information after uniformly charging the photosensitive member surface using a charger.
In the method of printing an image using the above-mentioned electrophotographic method on the basis of inputted image information, an electrostatic latent image consisting of plural printing levels and a nonprinting level is formed by exposing a photosensitive member surface that has been uniformly charged, and then the electrostatic latent image is developed to form a toner image. The larger the exposure quantity of light irradiated on the photosensitive member, the smaller the surface potential of the photosensitive member becomes. Therefore, when the exposure quantity is changed to form an electrostatic latent image in the form of a line image with the same beam diameter, the larger the exposure quantity, the larger the width of the latent image becomes, and the surface potential of the photosensitive member declines from the initial state.
In printing a line image, the line-width of an actual image becomes thicker than the line-width indicated by the image data because an electric field due to the spatial potential change of the electrostatic latent image itself is generated. In order to compensate for this phenomenon, a method of changing the exposure quantity to be irradiated on the photosensitive member according to the kind of printing image was proposed in a conventional two level printing method. In the Japanese patent publication No. 62-26621, for example, a method is disclosed wherein, when a printing image is recognized as a line image the exposure quantity is lowered when the photosensitive member surface is exposed to light modulated on the basis of image information. As a result, the thickening of lines after development is suppressed.
While the three level printing method has an advantage in that two color printing having no position dislocation can be performed at the same printing speed as a single color printer, because a latent image for two colors is formed by one exposure, changing the exposure quantity and thickening of the lines will occur for one color in the printing of the line image.
There is also another problem in that thinning of lines in the other color also occurs. The cause of this phenomenon is that, when exposing the photosensitive member surface in order to form the electrostatic latent image of a line, the surface potential of the photosensitive member declines by a width which is larger than the optical beam diameter. Because the surface potential of the photosensitive member declines depending on the exposure quantity, the degree of thickening of some lines and thinning of other lines depends on the exposure quantity. Therefore, it is impossible to print proper images using only a conventional control method wherein the exposure quantity is varied after recognizing the kind of the printing image.
Thus, it is an object of the present invention to provide an electrophotographic printing apparatus that can print desired images corresponding to image data by determining a proper exposure quantity and controlling the exposure quantity accordingly in a three level printing method.